(1) Field of the Invention
This invention relates to filled, physically enhanced composites of ultrahigh molecular weight polyethylene and more particularly to composites having high flex and tensile moduli and processes for the preparation thereof.
(2) Description of the Prior Art
Ultrahigh molecular weight polyethylene (UHMWPE) is known and commercially available. Lupton et al., in U.S. Pat. No. 3,944,536 describe preparation of UHMWPE having unusual toughness and rigidity by inducing crystallization of molten UHMWPE at high temperatures and pressures. The product has a crystalline melting point of 142.degree. to 148.degree. C. and a crystalline form characterized by the absence of chain fold spacings of 50-2000 Angstrom units and the presence of crystal spacings of about 10,000 Angstrom units indicating largely extended, unfolded polymer chains. U.S. Pat. No. 4,587,163 (Zachariades) discloses UHMWPE having isotropic semicrystalline morphology prepared by fusing the original powdered or melt-crystallized stock to a homogeneous melt and cooling under pressure. U.S. Pat. No. 4,655,769 (Zachariades) discloses a "pseudo-gel" of UHMWPE in a solvent, said polyethylene having a crystalline morphology comprising folded chain single crystals and extended chain "shish-kebab" fibrils.
U.S. Pat. Nos. 4,097,447 (Howard), 4,126,647 (Howard), 4,151,126 (Adelman and Howard), 4,187,210 (Howard) and 4,104,243 (Howard) disclose composites of polyolefins, including ultrahigh molecular weight polyethylene, with a variety of finely divided particulate inorganic filler compounds, including alumina, calcium carbonate, kaolinite clay, mica and conductive carbon, and organic filler materials such as polyacrylonitrile, wherein the composites comprise at least about 25% by weight of filler. The aforementioned patents also disclose methods of preparing composites which permit relatively high loadings of filler without sacrifice of essential physical properties In these processes ethylene is polymerized onto the surface of the filler particles such that the resultant composite is substantially free of polymer-free filler and of filler-free polymer. U.S. Pat. Nos. 4,330,573 and 4,234,659 disclose composites wherein a polyolefin having a molecular weight not less than 300,000 is polymerized onto a solid porous inorganic carrier material to provide composites wherein the inorganic carrier content is 50 to 99.5% by weight.
Eyrer et al., Kunstoffe German Plastics 77, 617-622 (1987), in an article entitled "Ultrahigh Molecular Weight Polyethylene for Replacement Joints", point out that the service life of certain joint replacements made of UHMWPE is limited. Analysis of the damage to over 250 explanted hip cups and tibial plateaus revealed a changed property profile which they explained by post-crystallization resulting from oxidative chain decomposition. They suggested optimizing the processing of polyethylene under higher pressure and higher temperature to increase the degree of crystallinity. The Eyrer et al. product displays a creep of above 5% at a compression of 1000 psi (6.9 N/nm.sup.2) for 24 hours at 37.degree. C. UHMWPE is a material of choice for use in prosthetic joints, particularly the load bearing hip, because of its high impact strength and inertness to blood. However, creep in the polymer has restricted use of UHMWPE in prosthetic parts.
It is an object of the present invention to provide homogeneous composites of enhanced UHMWPE with various filler materials wherein tensile modulus is significantly increased over conventional composites while essential strength properties and low creep are retained. It is a further object of the present invention to provide composites of UHMWPE that are useful in forming shaped articles including prosthetic articles. A feature of the present invention is a crystal morphology including folded and very highly extended molecular chains. It is an advantage of the present invention to provide relatively low cost composites of UHMWPE with filler materials. A still further object of the present invention is to provide lightly-filled composites of enhanced UHMWPE containing less than 10 weight % filler, said composites having many of the properties of unfilled enhanced UHMWPE. These and other objects, features and advantages of the present invention will become more readily apparent upon having reference to the following description of the invention.